Method of direct chill continuous casting

ABSTRACT

In the production of aluminium ingots, particularly round ingots of aluminium magnesium silicide alloys having &#39;&#39;&#39;&#39;hot-short&#39;&#39;&#39;&#39; characteristics, by the direct chill continuous casting process the casting speed is held below an experimentally determined crack propagating speed during the formation of a crack-free length of ingot, the casting speed then being increased to a speed above the crack propagating speed and below a higher experimentally determined crack initiating speed. The length of ingot formed at a speed below the crack propagating speed is of the order of 2-4 ingot diameters according to the diameter of the ingot (2-4 ingot thicknesses for rectangular ingots).

ilite cfiubbin [45] 9 W75 [54] METHOD or DIRE CHILL 0 1 0 3,713,479 1 1973 Bryson 164/89 CASTING FOREIGN PATENTS OR APPLICATIONS 1 lnvemorl Mm Gordon Mccubbina Kingston, 510,649 3/1955 Canada 164/283 MS Ontario, Canada [73] Assignee: Alcan Research and Development Primary Examiner-Francis S. Husar Limited, Montreal, Quebec, Canada Assistant ExaminerJohn E. Roethel Attorney, Agent, or FirmCooper, Dunham, Clark, [22] Flled. Jan. 21, 1974 Griffin & Moran [21] Appl. No.1 434,784

[57] ABSTRACT [30] Fm-e'gn Apphc'anonrrmnty Data In the production of aluminium ingots, particularly Jan. 24, 1973 United Kingdom 3699/73 round ingots f aluminium magnesium Sflicide alloys having hot-short characteristics, by the direct chili [52] US. Cl. 164/82; 164/283 MS Continuous casting process the Casting Speed is held [51] Int. Cl. 322d 11/12 below an experimentally determined crack propagat [58] meld of Search 164/82, 89, 283 MS ing speed during the formation of a crack-free length of ingot, the casting speed then being increased to a References Clted speed above the crack propagating speed and below a UNITED STATES PATENTS higher experimentally determined crack initiating 2,692,410 10/1954 Chadwick et a1. 164/89 p Th9 length of ingot fm'med at a $9999 9919! 2,708,297 5/1955 Zeigler 164/283 MS the crack propagating speed is of the order of 2-4 2,983,972 5/1961 Moritz 164/89 X ingot diameters according to the diameter of the ingot 5/1968 Gardner 164/283 M5 (2-4 ingot thicknesses for rectangular ingots). 3,441,079 4/1969 Bryson 1 164/89 3,672,431 6/1972 Bryson 164/82 11 Glaims, 2 Drawing Figures METHOD OF DIRECT CHILL CONTINUOUS CASTING The present invention relates to the production of light metal ingots, particularly aluminium (including aluminium alloy) ingots, by the direct chill casting process.

In the direct chill casting process, as conventionally practised, molten metal is introduced into a mould, the bottom end of which is initially closed by means of a stool, which may be lowered at a desired speed under the control of the operator to withdraw the ingot from the mould. When the metal is poured into the mould a solid skin forms at the periphery where the molten metal contacts the mould wall. After a predetermined depth of metal has been poured into the mould, the stool is lowered and heat is removed from the metal by application of coolant directly to the solidified surface skin of the metal exposed between the bottom margin of the mould and the stool. The rate at which the cast ing is produced is dependent upon the rate at which the stool is lowered while maintaining a substantially constant head of metal in the mould. In an alternative procedure molten metal is introduced into one end of a horizontally-arranged mould and withdrawn from the opposite end of the mould by a withdrawing member.

It is well known that with many aluminium alloys there is a tendency for cracks to form at the centre of the ingot as a result of the stresses which build up in the ingot as a result of the contraction of the metal at the centre of the ingot in its transition from the liquid to the solid state.

In general it is found that increase of the casting rate leads to the formation of centre cracks and, although many expedients have been suggested for increasing casting speed, the results obtained have been inconsistent. Whilst on some occasions it has been possible to cast sound, crackfree ingots at rates higher than those normally employed in commercial practice, on other occasions, cracks have been found to develop in ingots cast at the same speed.

It has now been appreciated that the explanation of these inconsistent results in the conditions employed in the transition from the slow casting speed normally employed at the beginning of each casting operation during the first few inches of the outward (vertical or horizontal) travel of the withdrawing member to the higher speed employed during the major part of the casting cycle.

It is well known that in the casting of aluminium ingots, in particular in casting aluminium alloys which exhibit hot-short cracking tendencies, there is a tendency for the first few inches of the ingot to exhibit a centre crack as a result of the very high cooling rates and as a consequence high stresses which result from the contact of the butt end of the ingot with the stool and with coolant, in addition to the normal cooling of the peripheral surface of the ingot.

Hot-short cracking may be defined as a tendency to tearing or cracking wherein the growth tip of the crack is at a zone of the ingot which is above the solidus temperature.

In the normal practice of direct chill casting this centre crack extends for only a few inches into the ingot and then terminates. Even so, it is frequently found that such cracks extend to a position beyond that at which the stool or horizontal withdrawing member has been accelerated to the full casting speed. In other words, the cracks have healed up after the full normal casting speed has been attained. In conventional casting procedures casting speeds are therefore limited to those which will permit cracks to heal within a short distance.

In accordance with the invention it has now been appreciated that the casting speed may be increased after the crack has healed to a speed, which if applied at an earlier instant in the travel of the stool would have led to an unsound ingot with a more or less continuous centre crack.

It has been realised that there is a maximum casting speed beyond which a cracked ingot will not heal as the casting operation continues. This speed is herein referred to as the crack propagating speed (C.P.S.); i.e. the minimum speed necessary to sustain the growth of a pre-existing crack. When the casting speed is decreased below the C.P.S. a pre-existing crack will heal.

However once the crack has healed and the drop has continued for a further distance to produce a length of sound ingot a fresh crack at the centre of the ingot will only be formed if the casting speed is raised to a value above the C.P.S. This higher speed is herein referred to as the crack initiating speed! (C.I.S.).

According to the present invention a process for the production of aluminium (including aluminium alloy) ingots by the direct chill casting method includes casting a length of ingot at a speed below the C.P.S. to develop a crack-free centre and thereafter raising the casting speed to a speed above the C.P.S. but below the C.I.S. The process is particularly applicable to the .production of round and square ingots.

Whilst in a preferred procedure the initial casting speed is maintained below the C.P.S. until a crack-free core has been developed, in some circumstances it might be desirable to employ a high initial casting speed in excess of the C.P.S. and accept the resultant crack, the casting speed then being decreased below the C.P.S. until the desired crack-free centre had been developed. The casting speed may then be again increased above the C.P.S. and held at such a speed (but below the C.I.S.) during the casting of the remainder of the ingot.

In carrying out the process it is preferred to utilise a speed substantially below the C.P.S. during the first few inches of the drop. For ingot. sizes in the range of 4-9 inches in diameter or 4-9 inches square, during the first length of the drop equal to about one-half an ingot diameter (or square side) it is preferred to employ a speed below 4 inches/min. For ingots of larger or smaller sizes, this speed will be somewhat smaller or larger, respectively. Thereafter the speed is raised over the distance of about another /2 ingot diameter to a speed slightly below the C.P.S., such as /2-l in./min. less than the C.P.S., the drop being continued at such speed until a crack-free ingot length of at least about 1 ingot diameter and preferably equal to at least about 2 /2 ingot diameters, has been developed and thereafter slowly increasing the casting :speed to a speed intermediate the C.P.S. and the C.I.S. The maximum casting speed employed is preferably /24 in./min. less than the C.l.S. A speed above the C.P.S. is usually reached after casting about 3 /2 diameters of ingot. For ingot sizes 9 inches and 4 inches the crack-free length to be cast before increasing to the speed between C.P.S. and C.I.S. should be about 2 and 4 diameters, respectively. Where the ingot is of rectangular section this distance should be related to the minimum transverse dimension of the ingot (its thickness) in place of the diameter.

Preferably the transition from the casting speed below the CPS. to the speed between the CPS. and the C.l.S. is carried out in progressive increments of about one-half in./min., each speed increment being applied slowly, for example in -30 seconds. Whilst for convenience of operation the speed is increased in separate increments at intervals of, say 1 minute, it may in some circumstances be possible to increase the casting speed progressively from the first speed below the CPS. to the speed between the CPS. and the C.l.S. at a substantially constant rate of acceleration of, for example As-5 in./min. per minute, so that the casting speed is increased from a speed slightly below the C.P.S. to the final casting speed in /2-15 minutes.

The procedure of the present invention is of particular value in the production of round ingots formed of those alloys which exhibit sensitivity to hot-short cracking. A number of commonly used alloys in Al-Mn, Al- Mg, Al-Cu and Al-Zn-Mg series and most particulary alloys in Al-lvlg-Si series, which are widely employed for the production of extrusions, exhibit sensitivity to hot-short cracking. In particular the process of the present invention has been found to be of value in casting Al-Mg-Si alloys having the respective percentage contents of magnesium and silicon as follows:

Mg; 0.45-0.9; Si 020-06; Mg 0.40.8; Si 0.7-1.3; Mg 0.8-1.2; Si 0.400.8; Mg 035-08; Si 0.300.7.

These are extensively used commercial aluminium alloys.

The CPS. and C.l.S. for the production of ingots from a given alloy depend on a number of factors, which arise in part from the casting conditions and in part from the metal composition and condition.

Thus the temperature at which the metal is poured into the mould, the type of float and dip tube used for controlling the metal level in the mould, the mould diameter or transverse dimensions and the method of applying coolant to the ingot surface all influence the value of the C.P.S. and the C.l.S.

In addition to the main alloying elements, the cleanliness of the metal and the quantity of iron present in the aluminium and the presence of elements which affect grain size also exert an important influence on these values. in general the presence of both iron and grain refiners increases the CPS. and the C.l.S. values.

The determination of the values of the CPS. and C.l.S., because of the large number of variables on which they are dependent, can only be made empirically by casting an ingot from a melt of the alloy. For example, with a 7-inch diameter ingot of AA6063 alloy the casting speed should be increased by steps from an initial speed of, say, 4 ins./min., progressively by increments of, for example, one-half in./min. as discussed above, at intervals of cast length of, say, 24 inches until the expected C.l.S. is substantially exceeded and then progressively decreasing the casting speed in like manner. For ingots of smaller diameter and/0r alloys less prone to hot short cracking, larger initial trial speeds may be used, with the same increments; while for ingots of larger diameters and/or alloys more hot-short prone, lower initial trial speeds and smaller imcrements are warranted. By recording the casting speed at successive positions along the ingot, subsequent ultrasonic inspec tion of the ingot will permit determination of the casting speed at which the centre crack commenced (oth erwise than at the butt end) in a significant number of ingots. This will determine the C.l.S.

Determination of the casting speed at which the centre crack ceased during the decrease of the casting speed will indicate the value of the CPS. and, more particularly, a speed below the CPS. at which the drop may be carried out at the beginning of the casting cycle in order to develop a sound centre.

As an example of the benefits attainable by employment of the invention it is found that in the production of 7-inch diameter extrusion ingots of aluminium magnesium silicide alloy containing 0.4-0.9% Mg, 0.3-0.7% Si, 02-03% Fe and 0.005-0.l5% Ti the normal maximum casting speed for the production of sound ingots is about 5 in./min. after casting the first few inches at a speed of 3-4 in./rnin. When that procedure is followed it is found that there is frequently a centre crack extending for about 8 inches upwardly from the butt end. Following the procedure of the pres ent invention an initial length of the ingot, for example 24 inches, is cast at 5 in./min. (after casting the first few inches at a speed of 3-4 inches/min. as above) to develop a length of ingot having a crack-free centre. It was found that the casting speed could then be increased slowly to 6 /2 in./min. in the manner indicated above without a centre crack being initiated. If the same speed had been employed at the beginning of the casting process, i.e. before the butt crack had healed, the ingot would have been cracked from end to end.

The invention will now be described by way of further example with reference to the accompanying drawings, in which:

Fifi. 'l is a diagrammatic part-sectional view of a continuous casting apparatus, and

FIG. 2 is a sectional view on the line 2-2 of FIG. 1.

FIG. l shows an axially vertical annular mould 10 to which molten aluminium ll is supplied for casting an ingot l2. The mould 10 is constructed of a metal (such as aluminium) suitable for casting aluminium and has an inner wall 14 which defines an axially vertical casting zone.

Surrounding the outer surface of the mould wall 14 is a cooling jacket 15 defining an annular chamber completely laterally surrounding the casting zone. The chamber is supplied with cooling water, for chilling the mould l4, and is provided with an outlet slot 22 through which water is applied to the surface of the growing ingot 12 in the conventional manner.

At the start of a casting operation, the lower end of the casting zone is closed by a stool 18 which is carried on a platform 19 supported on a hydraulic ram 20. As molten aluminium in the casting zone solidifies, the stool l8 and platform 19 are lowered by operation of the hydraulic ram 20 at a speed controlled by the operator.

Molten aluminium is supplied to the mould from a launder 26 positioned above the mould l0 and having an axially vertical dip tube 28 extending coaxially into the mould. The lower end of dip tube 28 opens beneath the level of the molten aluminium 11 in the casting zone; this prevents surface turbulence in the casting zone pool.

A float 30 surrounds the lower end of dip tube 28 and is buoyantly supported in the molten metal ll. The float comprises a relatively thick rigid disc-shaped body 3i having a central bore 32 of slightly larger diameter than the dip tube 28. The lower surface of body 31 has a central recess 34 into which the bore 32 opens. A rigid baffle portion 36 of the float extends across the recess'34, immediately beneath the bore 32. The baffle portion 36 has an upper surface wider than the internal diameter of the dip tube 28. When the dip tube 28 ex tends downwardly through bore 32, its open lower end is directly above the baffle. The molten aluminium enters through the dip tube 28 and is directed laterally by baffle 36 towards the periphery of the mould.

it should be understood that since the operating level of the molten metal lll in the mould should be maintained above the lower end of the dip tube 28., and since the upper surface of the baffle 36 is ordinarily below the end of the dip tube to permit discharge of molten metal, the buoyant properties of the float should provide this desired space relation of dip tube and baffle when the molten metal is at the proper level in the mould.

A pair of wire supports 38 are carried by the float, and are bent outward to project over opposite sides of the top wall of the mould to prevent descent of the float beyond a predetermined level.

In operating the apparatus in accordance with the present invention the ram 20 is initially positioned so that the stool lll closes the lower end of the casting zone. Cooling water is continuously supplied to the mould cooling jacket for chilling the mould wall 14 and molten aluminium supplied from the launder 26 flows downwardly by gravity through the dip tube 28 to till the casting zone with molten aluminium ll to the desired level. The hydraulic ram 24 is then operated to lower the stool 18 at a low initial speed, such as 3-4 ins/min. for ingots in the size range of 49 inch diameter, in the conventional manner whilst the stool travels through a distance, d typically 2-5 inches for these size of ingots.

After the stool has descended by the distance ti the operator slowly increases the dropping speed of the stool as it descends through a further distance 41;, typically 25 inches to a speed slightly below the CPS. After the stool has descended by this further distance d a length of ingot corresponding to about 2 /2 ingot diameters is cast at this same speed, following which the operator gradually increases the dropping speed to a speed exceeding the C.P.S., but below the C.l.S.

In one example the CPS. and the C.l.S. ofa particular Al-Mg-Si alloy was determined by the procedure set out above. The CPS. was found to be about 66 /2 ins/min. and the C.I.S. was found to be about 8 ins./-

min.

The following casting procedure was established for casting 7inch diameter ingots from this alloy:

4 in./min. for 5 inches increase to 6 in./min. during following 6 inches hold at 6 in./min. for inches increase to 7 /2 in./rnin. and hold at 7 /2 in/min. for

remaining 80 inches.

Two ingots cast by this procedure were found to be sound.

By contrast the following casting procedure was fol lowed:

4 injmin for 4 inches 7 /2 in/min. for remaining 75 inches Two ingots cast by this procedure was found to be cracked end to end, because the casting speed was increased from below the CPS. to above the C.P.S. before the centre crack had healed.

l claim:

1;. A method of producing light metal ingots by the direct chill casting process in which molten metal is introduced into one end of an openended mould and a solidified ingot is withdrawn from the opposite end of the mould, fluid coolant being applied directly to the solidified surface of the ingot emerging from the mould, in which, in the course of forming the ingot, at one stage the ingot is withdrawn from the mould at a speed below the crack propagating speed for a sufficient distance to develop a crack-free centre in the emerging ingot, the speed of ingot withdrawal then being increased to a speed above the crack propagating speed and below the crack initiating speed.

2. A method according to claim 1 in which the emerging ingot is withdrawn from the mould at a speed below the crack propagating speed until a length of crack-free ingot equal to at least about I ingot diameter or 1 ingot minimum transverse dimension (for a round ingot or rectangular ingot respectively) has been formed before the speed of ingot withdrawal is increased to a speed above the crack-propagating speed and below the crack-initiating speed.

3. A method according to claim 1 in which an aluminium alloy is cast to form a round ingot or square ingot of 49 inches diameter or minimum transverse dimension, the ingot withdrawal speed being held at a speed below the crack propagating speed during the withdrawal of a length of ingot of at least 2 /2 ingot diameters or 2 /2 minimum transverse dimensions.

4. A method according to claim 3 in which the aluminium alloy is an aluminiumymagnesium silicon alloy having hot-short characteristics.

5. A method according to claim 4 in which the alloy contains Mg OAS-0.9% and Si O.20O.6%.

6. A method according to claim 4 in which the alloy contains Mg GAO-0.8% and Si 0.7-1.3%.

7. A method according to claim 4 in which the alloy contains Mg 0.8-1.2% and Si GAO-0.8%.

8. A method according to claim 4 in which the alloy contains Mg O.350.8% and Si O.30-O.7%.

I A method according to claim l in which the ingot withdrawal speed is /2-1 inch/min. less than the crack initiating speed during the withdrawal of at least a major part of the crack-free length of the ingot.

W. A method according to claim 1 in which an aluminium alloy is cast to form a round ingot or square ingot of a diameter or minimum transverse dimension of less than 4 inches, the ingot withdrawal speed being held at a speed below the crack propagating speed during the withdrawal of a length of ingot of at least 4 ingot diameters or 4 ingot minimum transverse dimensions respectively.

ll. A method according to claim 1 in which an aluminium alloy is cast to form a round ingot or square ingot of a diameter or minimum transverse dimension of more than 9 inches, the ingot Withdrawal speed being held at a speed below the crack propagating speed during the withdrawal of a length of ingot of at least 2 ingot diameters or 2 ingot minimum transverse dimensions respectively.

:k :r :r 

1. A method of producing light metal ingots by the direct chill casting process in which molten metal is introduced into one end of an open-ended mould and a solidified ingot is withdrawn from the opposite end of the mould, fluid coolant being applied directly to the solidified surface of the ingot emerging from the mould, in which, in the course of forming the ingot, at one stage the ingot is withdrawn from the mould at a speed below the crack propagating speed for a sufficient distance to develop a crackfree centre in the emerging ingot, the speed of ingot withdrawal then being increased to a speed above the crack propagating speed and below the crack initiating speed.
 2. A method according to claim 1 in which the emerging ingot is withdrawn from the mould at a speed below the crack propagating speed until a length of cRack-free ingot equal to at least about 1 ingot diameter or 1 ingot minimum transverse dimension (for a round ingot or rectangular ingot respectively) has been formed before the speed of ingot withdrawal is increased to a speed above the crack-propagating speed and below the crack-initiating speed.
 3. A method according to claim 1 in which an aluminium alloy is cast to form a round ingot or square ingot of 4-9 inches diameter or minimum transverse dimension, the ingot withdrawal speed being held at a speed below the crack propagating speed during the withdrawal of a length of ingot of at least 2 1/2 ingot diameters or 2 1/2 minimum transverse dimensions.
 4. A method according to claim 3 in which the aluminium alloy is an aluminium magnesium silicon alloy having hot-short characteristics.
 5. A method according to claim 4 in which the alloy contains Mg 0.45-0.9% and Si 0.20-0.6%.
 6. A method according to claim 4 in which the alloy contains Mg 0.40-0.8% and Si 0.7-1.3%.
 7. A method according to claim 4 in which the alloy contains Mg 0.8-1.2% and Si 0.40-0.8%.
 8. A method according to claim 4 in which the alloy contains Mg 0.35-0.8% and Si 0.30-0.7%.
 9. A method according to claim 1 in which the ingot withdrawal speed is 1/2 -1 inch/min. less than the crack initiating speed during the withdrawal of at least a major part of the crack-free length of the ingot.
 10. A method according to claim 1 in which an aluminium alloy is cast to form a round ingot or square ingot of a diameter or minimum transverse dimension of less than 4 inches, the ingot withdrawal speed being held at a speed below the crack propagating speed during the withdrawal of a length of ingot of at least 4 ingot diameters or 4 ingot minimum transverse dimensions respectively.
 11. A method according to claim 1 in which an aluminium alloy is cast to form a round ingot or square ingot of a diameter or minimum transverse dimension of more than 9 inches, the ingot withdrawal speed being held at a speed below the crack propagating speed during the withdrawal of a length of ingot of at least 2 ingot diameters or 2 ingot minimum transverse dimensions respectively. 